This is a topic I really haven't given any thought to at all. I went day hiking the other day and forgot my water bottle. I was lucky enough to hike by a stream but stopped and started thinking before I drank. It's been sitting in the back of my head ever since.

I used a MSR mini works filter for years and still use it when kayak camping but when backpacking I now use a Steripen uv light. It isn't many more $$$ than a good filter and mine weighs 4 oz. with batteries. It also works on viruses which, while pretty much a non-issue in this country, gives me peace of mind while traveling in Mexico and Central America.

I heard an interview with Cody Lundin a few days ago. He says that you can sterilize water by putting it in a clear plastic container and leaving it in the sun. The UV is supposed to sterilize the water. I can't remember the length of time, but it wasn't very long, perhaps 30 minutes to an hour. He did indicate that this works in the southern desert where he lives (Arizona), and may not work farther north.

Micropur tablets - lightest/easiest things available (I think). I bring a 2 liter platy, and a 1 liter platy...this allows me to sterilize a liter at a time while continuously hydrating from my 2L platy.

Perhaps this is a bit too radical for some, but I have used this approach for several trail years with no problem:

For sources with low potential for contamination, I strain the floaties with a piece of nylon hose and treat with 2 drops of chlorine bleach per liter. Relatively safe sources are springs with deep contributing areas and the upper reaches of streams that do not flow near trails, roads, settlements or campgrounds if and only if the streams do not produce the durable bubbles that indicate a high level of dissolved organic matter.

I do not consider this alone to be adequate where the potential for giargia is high: in beaver country where streams can be the outfall of beaver ponds, and in areas with a lot of human traffic. In those cases any .2 micron filter will work because giardia cysts are so large. I chemically treat filtered water with bleach. Usually, however, I just boil such suspect water; since I travel long distances and usually get to pick and chose among water sources, I seldom have to settle on a suspect source and rarely bother to carry a filter.

Does anyone rely on pasteurization for water treatment? I've had boiling for 5 minutes recommended to me before - which is overkill when almost all the bugs get killed off in less than a minute at 65 C.

I think even the slowest-acting of water critters won't cause any effects until 7 days, so wait and see. After 10 days and no effects, you can rejoice.

For those "oops!" cases, I have a 0.75 oz. McNett survival straw, an $8 water filter that you drink through, in my kit all the time. I've never used it in the field, but I bet with some fannagling someone could rig this as a main filter. Only good for 20 gallons or so. I also carry a few tabs of MicroPur, just in case.

If the water will be clear and free-flowing, I use the (original) Steri-Pen with the pre-filter (connected to a 1L Nalgene Cantene). With 4 lithium batteries, it weighs 7 oz. (Although the Adventurer is smaller and 3 oz. lighter, I couldn't justify the switch.) I carry a dozen MicroPur tabs as backup.

If the water will be icky and murky, or if there's the chance of herbicide/pesticide runoff, it's back to the General Ecology First Need filter. At 16 oz. it's as heavy as they get, but I swear this thing can turn nuclear sludge into Perrier. I've never had the filter weigh more than 16 oz. even after heavy use, so it doesn't hold water as is purported of other filters.